In wireless orthogonal frequency division multiple access (OFDMA) communication systems, the transmit power of each mobile station needs to be maintained at a certain level to achieve desired channel quality and to maximize system capacity. Because radio resources in wireless OFDMA systems are partitioned into subcarriers within each radio frequency (RF) carrier, radio signals are transmitted from each mobile station to its serving base station in data streams using different subcarriers. As a result, the transmit power per data stream and per subcarrier for the current transmission depends on the target uplink signal to interference plus noise power ratio (SINR) received by the serving base station, and the noise and interference level per subcarrier at the serving base station. In a Close Loop Power Control (CLPC) mechanism, the transmit power of each mobile station is controlled by a Transmit Power Control (TPC) command transmitted from the serving base station. The serving base station periodically monitors uplink channel quality of the mobile stations, and in response, sends TPC commands for periodic transmit power update for the mobile stations.
Uplink channel quality of the mobile stations can be derived or estimated by the serving base station based on downlink channel measurements performed by the mobile stations. In Time Division Duplex (TDD) systems, uplink channel quality can be derived from downlink channel measurement due to channel reciprocity. In Frequency Division Duplex (FDD) systems, uplink channel quality can also be estimated from downlink channel measurement by exploiting channel correlation between downlink and uplink channels.
In one embodiment of an IEEE 802.16m system, the serving base station first configures the duration and periodicity of a fast feedback channel (FFBCH) for a mobile station. The mobile station then reports its downlink channel information via the configured FFBCH. The serving base station estimates the uplink channel quality, e.g., uplink SINR, based on the received FFBCH. In response to the estimated uplink channel quality, the serving base station detects a channel variation and then transmits a power control command to track the channel variation by delivering a power control A-MAP-IE inside the power control Advanced-MAP (PC-A-MAP) in a downlink frame. Based on the received PC-A-MAP-IE, the mobile station identifies the power control command and adjusts the transmit power level accordingly. When there are multiple mobile stations in the wireless OFDMA system, it is likely that multiple power control commands for multiple mobile stations will be aggregated within the same PC-A-MAP.
In order to identify its power control command inside the PC-A-MAP, a mobile station needs to know the address of the power control command, i.e., PC-A-MAP-IE inside the PC-A-MAP. However, it remains a challenge to provide a PC-A-MAP-IE indexing to the mobile stations for efficient and effective transmit power control.